Peroxide-crosslinkable fluoroelastomers are generally obtained by introducing halogen atoms contributing to peroxide-crosslinking into polymer chain ends, or by introducing peroxide-crosslinkable halogen atoms into polymer side chains. Iodine or bromine atoms are introduced as halogen atoms.
In order to increase the crosslinking density of a high-molecular-weight elastomer, it is necessary to introduce a halogen atom-containing side chain, and this is achieved by copolymerization with a monomer having a peroxide-crosslinkable halogen atom.
Monomer compounds in which the carbon atoms forming a double bond are directly bonded to iodine atoms are known; however, there are few examples of monomer compounds in which an iodine atom is bonded to the end of the long side chain. The synthesis of such monomer compounds using a known method has a long synthesis route and results in a poor yield; thus, productivity is extremely low (see Patent Documents 1 and 2).
Moreover, the synthesis of terminally iodized fluorine-containing monomers generally requires multistep reactions, each reaction of which is very troublesome. On the other hand, many unsaturated compounds having a bromine atom at the end of the long side chain are known; however, terminal bromine atoms have less reactivity than terminal iodine atoms.
Furthermore, when an elastomer having a long side chain contains a peroxide-crosslinkable halogen atom in its side chain, other side chains that are not contributed in crosslinking are presumably steric hindrance, preventing crosslinking reactions. Consequently, the vulcanizate physical properties and compression set characteristics of the resulting cross-linked products are not satisfactory.